Method and apparatus for performing handoff in communication systems

ABSTRACT

A communication system including a closed loop power control system. Prior to allowing a handoff to a new base station, the subscriber station verifies that its reverse link signal is being received by the destination base station with sufficient energy. The determination is made based on the received reverse link power control commands from the base station. Moreover, a handoff may be forced when the base station providing the best forward link signal in not receiving the reverse link signal from the subscriber station with sufficient energy.

The present application for patent is a continuation of and claimspriority to U.S. patent application Ser. No. 11/668,618 (“the '618application”), titled “METHOD AND APPARATUS FOR PERFORMING HANDOFF IN AHIGH SPEED COMMUNICATION SYSTEM,” filed Jan. 30, 2007, now allowed. The'618 application is a continuation of and claims priority to U.S. patentapplication Ser. No. 09/434,314 (“the '314 application”), titled “METHODAND APPARATUS FOR PERFORMING HANDOFF IN A HIGH SPEED COMMUNICATIONSYSTEM,” filed Nov. 4, 1999, now U.S. Pat. No. 7,206,580. Both the '618application and the '314 application are assigned to the assignee of thepresent application, and are expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for performing handoff in a wireless communicationsystem.

2. Description of the Related Art

It has become very important for service providers to be able to providehigh speed wireless services to their customers. A high speed wirelesscommunication system is disclosed in U.S. Pat. No. 6,574,211, (the '211patent), issued Jun. 3, 2003, entitled, “METHOD AND APPARATUS FOR HIGHERRATE PACKET DATA TRANSMISSION,” which is assigned to the assignee of thepresent invention and incorporated by reference herein. In the '211patent, the base station transmits to subscriber stations by sendingframes that include a pilot burst time multiplexed in to the frame andtransmitted at a rate based on channel information transmitted from thesubscriber station to the base station. This system is optimized for thewireless transmission of digital data.

Code Division Multiple Access or CDMA has proven itself to be thepredominant choice for wireless service providers because of its highspectral efficiency. One such CDMA communication system is described inthe “TIA/EIA/IS-95 Subscriber station-Base Station CompatibilityStandard for Dual-Mode Wideband Spread Spectrum Cellular System,”hereinafter referred to as the IS-95 standard. The IS-95 CDMA systemallows for voice and data communications between users over aterrestrial link. The use of CDMA techniques in a multiple accesscommunication system is disclosed in U.S. Pat. No. 4,901,307, entitled“SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE ORTERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEMAND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONESYSTEM,” both assigned to the assignee of the present invention andincorporated by reference herein.

In this specification, base station refers to the hardware with whichthe subscriber stations communicate. Cell refers to the hardware or thegeographic coverage area, depending on the context in which the term isused. A sector is a partition of a cell. Because a sector of a CDMAsystem has the attributes of a cell, the teachings described in terms ofcells are readily extended to sectors.

In the CDMA system, communications between users are conducted throughone or more base stations. A first user on one subscriber stationcommunicates to a second user on a second subscriber station bytransmitting data on the reverse link to a base station. The basestation receives the data and can route the data to another basestation. The data is transmitted on the forward link of the same basestation, or a second base station, to the second subscriber station. Theforward link refers to transmission from the base station to asubscriber station and the reverse link refers to transmission from thesubscriber station to a base station. In IS-95 systems, the forward linkand the reverse link are allocated separate frequencies.

The subscriber station communicates with at least one base stationduring a communication. CDMA subscriber stations are capable ofcommunicating with multiple base stations simultaneously during softhandoff. Soft handoff is the process of establishing a link with a newbase station before breaking the link with the previous base station.Soft handoff minimizes the probability of dropped calls. The method andsystem for providing a communication with a subscriber station throughmore than one base station during the soft handoff process are disclosedin U.S. Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HANDOFF IN ACDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the presentinvention and incorporated by reference herein. Softer handoff is theprocess whereby the communication occurs over multiple sectors which areserviced by the same base station. The process of softer handoff isdescribed in detail in U.S. Pat. No. 5,933,787, entitled “METHOD ANDAPPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASESTATION,” issued Aug. 3, 1999, assigned to the assignee of the presentinvention and incorporated by reference herein

A significant difference between voice services and data services is thefact that the former imposes stringent and fixed delay requirements.Typically, the overall one-way delay of speech frames must be less than100 msec. In contrast, the data delay can become a variable parameterused to optimize the efficiency of the data communication system.Specifically, more efficient error correcting coding techniques whichrequire significantly larger delays than those that can be tolerated byvoice services can be utilized. An exemplary efficient coding scheme fordata is disclosed in U.S. Pat. No. 5,933,462, entitled “SOFT DECISIONOUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,” issuedAug. 3, 1999, assigned to the assignee of the present invention andincorporated by reference herein.

Another significant difference between voice services and data servicesis that the former requires a fixed and common grade of service (GOS)for all users. Typically, for digital systems providing voice services,this translates into a fixed and equal transmission rate for all usersand a maximum tolerable value for the error rates of the speech frames.In contrast, for data services, the GOS can be different from user touser and can be a parameter optimized to increase the overall efficiencyof the data communication system. The GOS of a data communication systemis typically defined as the total delay incurred in the transfer of apredetermined amount of data, hereinafter referred to as a data packet.

Yet another significant difference between voice services and dataservices is that the former requires a reliable communication linkwhich, in the exemplary CDMA communication system, is provided by softhandoff. Soft handoff results in redundant transmissions from two ormore base stations to improve reliability. However, this additionalreliability is not required for data transmission because the datapackets received in error can be retransmitted. For data services, thetransmit power used to support soft handoff can be more efficiently usedfor transmitting additional data.

The parameters which measure the quality and effectiveness of a datacommunication system are the transmission delay required to transfer adata packet and the average throughput rate of the system. Transmissiondelay does not have the same impact in data communication as it does forvoice communication, but it is an important metric for measuring thequality of the data communication system. The average throughput rate isa measure of the efficiency of the data transmission capability of thecommunication system.

It is well known that in cellular systems thesignal-to-noise-and-interference ratio C/I of any given user is afunction of the location of the user within the coverage area. In orderto maintain a given level of service, TDMA and FDMA systems resort tofrequency reuse techniques, i.e., not all frequency channels and/or timeslots are used in each base station. In a CDMA system, the samefrequency allocation is reused in every cell of the system, therebyimproving the overall efficiency. The C/I that any given user'ssubscriber station achieves determines the information rate that can besupported for this particular link from the base station to the user'ssubscriber station. Given the specific modulation and error correctionmethod used for the transmission, which the present invention seek tooptimize for data transmissions, a given level of performance isachieved at a corresponding level of C/I. For an idealized cellularsystem with hexagonal cell layouts and utilizing a common frequency inevery cell, the distribution of C/I achieved within the idealized cellscan be calculated.

The C/I achieved by any given user is a function of the path loss, whichfor terrestrial cellular systems increases as r³ to r⁵, where r is thedistance to the radiating source. Furthermore, the path loss is subjectto random variations due to man-made or natural obstructions within thepath of the radio wave. These random variations are typically modeled asa log normal shadowing random process with a standard deviation of 8 dB.

The obtained C/I distribution can only be achieved if, at any instant intime and at any location, the subscriber station is served by the bestbase station which is defined as that achieving the largest C/I value,regardless of the physical distance to each base station. Because of therandom nature of the path loss as described above, the signal with thelargest C/I is not always transmitted by the base station closest to thesubscriber station. In contrast, if a subscriber station was tocommunicate only via the base station of minimum distance, the C/I canbe substantially degraded. It is therefore beneficial for subscriberstations to communicate to and from the best serving base station at alltimes, thereby achieving the optimum C/I value. It can also be observedthat the range of values of the achieved C/I, in the above idealizedmodel, is such that the difference between the highest and lowest valuecan be as large as 10,000. In practical implementation the range istypically limited to approximately 1:100 or 20 dB. It is thereforepossible for a CDMA base station to serve subscriber stations withinformation bit rates that can vary by as much as a factor of 100, sincethe following relationship holds:

$\begin{matrix}{{R_{b} = {W\frac{\left( {C/I} \right)}{\left( {E_{b}/I_{o}} \right)}}},} & (1)\end{matrix}$

where R_(b) represents the information rate to a particular subscriberstation, W is the total bandwidth occupied by the spread spectrumsignal, and E_(b)/I_(o) is the energy per bit over interference densityrequired to achieve a given level of performance. For instance, if thespread spectrum signal occupies a bandwidth W of 1.2288 MHz and reliablecommunication requires an average E_(b)/I_(o) equal to 3 dB, then asubscriber station which achieves a C/I value of 3 dB to the best basestation can communicate at a data rate as high as 1.2288 Mbps. On theother hand, if a subscriber station is subject to substantialinterference from adjacent base stations and can only achieve a C/I of−7 dB, reliable communication can not be supported at a rate greaterthan 122.88 Kbps. A communication system designed to optimize theaverage throughput will therefore attempt to serve each remote user fromthe best serving base station and at the highest data rate R_(b) whichthe remote user can reliably support. The data communication system ofthe present invention exploits the characteristic cited above andoptimizes the data throughput from the CDMA base stations to thesubscriber stations.

SUMMARY OF THE INVENTION

The present invention resides in a communication system, apparatus, andmethod for performing handoff in a wireless communication system, whichtakes into account the ability of a base station to receive the reverselink transmissions from the subscriber station.

The subscriber station receives the pilot signal and reverse link powercontrol commands from all of the base stations in its Active Set. Thesubscriber station uses the received pilot signal for coherentdemodulation of the forward link traffic signal and for determining thestrength of the signal from each base station. In the exemplaryembodiment, the power control commands from each base station direct thesubscriber station to increase or decrease its transmission energy bypredetermined amounts. In the exemplary embodiment, the subscriberstation only increases its transmission energy when all base stations inthe Active Set request the subscriber station to increase itstransmission energy.

In the exemplary embodiment of the communication system described in theaforementioned U.S. Pat. No. 6,574,211, the high speed forward linktraffic data is transmitted from only one base station. That is to say,the forward link traffic is not provided in soft handoff. This is adesirable constraint from the perspective of overall system capacity,because the redundant transmission required for soft handoff greatlyimpairs overall system capacity. In the exemplary embodiment, thesubscriber station measures the signal energy of signals received fromeach of the base stations in the Active Set of the subscriber stationand sends a data request control (DRC) signal indicating which basestation is transmitting the strongest received signal. In addition, theDRC signal indicates a data rate which the subscriber station selectsbased on strength of the received signal from the selected base station.

In the exemplary embodiment of the present invention, the subscriberstation stores an indication of the mix of power control commandstransmitted by each base station. That is, for each base station anindicator respecting relative number of commands requesting an increasein transmission energy versus the number of power control commandsrequesting a decrease in transmission energy is stored. This statisticcan be generated by a filtering of the power control commands from eachbase station. For example, an infinite impulse response filter can beused to perform an averaging of the commands. The implementation ofaveraging filters is well known in the art.

In an alternative embodiment, the subscriber station stores the powercontrol commands from each base station. In a second alternativeembodiment, the subscriber station stores an indication of the number ofconsecutive or nearly consecutive requests to increase transmissionenergy from each base station. A series of requests to increasetransmission energy indicates that the base station is not receiving thereverse link signal.

The subscriber station makes an initial selection of the base station totransmit forward link data to it. In the exemplary embodiment, thesubscriber station measures the energy of a time multiplexed pilotsignal from each base station and selects the base station with thehighest chip energy to interference (C/I) when including all multipathcomponents from each base station. In the exemplary embodiment, thesubscriber station includes a RAKE receiver that separately demodulatesthe multipath components of signals from each base station. An exemplaryembodiment of a RAKE receiver is described in U.S. Pat. No. 5,103,390.

The subscriber station determines whether the selected base stationrequires a handoff. That is to say, whether the selected base station isthe same as the base station selected to transmit in the last frameinterval.

If the selected base station does require a handoff, then the subscriberuses the method of the present invention to determine if the selectedbase station is receiving its reverse link transmissions. In theexemplary embodiment, the subscriber station makes this determination bylooking at the history of reverse link power control commandstransmitted by the selected base station. A sufficient number of powercontrol commands by a given base station requesting the subscriberstation to decrease its transmission energy indicates that the reverselink signal is being received by the base station with sufficientenergy. It will be understood that other methods of performing thisanalysis are equally applicable, for example the base stations couldintermittently transmit a message indicating the average quality of thereceived reverse link signal.

If the subscriber station determines that its reverse link signal isbeing received with sufficient energy by the selected base station, thenthe handoff is permitted. The subscriber station transmits a messageindicative of the selected base station and the rate (or change intransmission power) requested to transmit to the subscriber station.

If the subscriber station determines that its reverse link signal is notbeing received with sufficient energy by the selected base station, thenthe handoff is inhibited. In the exemplary embodiment, the subscriberstation selects an alternative base station for transmission of forwardlink traffic data which is receiving its reverse link transmissions withsufficient energy. The subscriber station transmits a message indicativeof the alternative base station and the rate requested to transmit tothe subscriber station. The rate requested is based on the strength ofthe received pilot signal from the alternative base station.

If a handoff is not necessary, then the subscriber station againdetermines whether the selected base station (which is the base stationselected to transmit to the subscriber station in the last frame) isreliably receiving its reverse link signal. If the subscriber stationdetermines that its reverse link signal is being received by theselected base station with sufficient energy, then the subscriberstation transmits a message indicative of the selected base station andthe rate requested to transmit to the subscriber station.

If the subscriber station determines that its reverse link signal is notbeing received with sufficient energy by the selected base station, thena handoff is forced. The subscriber station selects an alternative basestation for transmission of forward link traffic data, which isreceiving its reverse link transmissions with sufficient energy. Thesubscriber station transmits a message indicative of the alternativebase station and the rate requested to transmit to the subscriberstation. The rate or power requested is based on the strength of thereceived pilot signal from the alternative base station.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a flowchart illustrating an exemplary method of performing ahandoff;

FIG. 2 is a basic diagram illustrating an exemplary embodiment of acommunication system;

FIGS. 3A and 3B is a block diagram illustrating an exemplary embodimentof a base station;

FIGS. 4A and 4B are diagrams illustrating an exemplary embodiment of aframe structure and slot structure; and

FIG. 5 is a block diagram illustrating an exemplary embodiment of asubscriber station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Overview

Referring to FIG. 1, in block 100, the subscriber station receives thepilot signal and reverse link power control commands from all of thebase stations in its Active Set. The subscriber station uses thereceived pilot signal for coherent demodulation of the forward linktraffic signal and for determining the strength of the signal from eachbase station. In the exemplary embodiment, the power control commandsfrom each base station direct the subscriber station to increase ordecrease its transmission energy. In the exemplary embodiment, thesubscriber station only increases its transmission energy if no basestations in the Active Set request the subscriber station to decreaseits transmission energy.

In the exemplary embodiment of the communication system described in theaforementioned U.S. Pat. No. 6,574,211, the high speed traffic data istransmitted from only one base station. That is to say, the forward linktraffic is not provided in soft handoff. This is a desirable constraintfrom the perspective of overall system capacity. In the exemplary, thesubscriber station measure the signal energy of signals from each of thebase stations in the Active Set of the subscriber station and sends adata request control (DRC) signal indicating which base station istransmitting the strongest received signal. In addition, the DRC signalindicates a data rate which the subscriber station selects based onstrength of the received signal from the selected base station.

In block 102, the subscriber station stores the reverse link powercontrol commands from each base station in memory. In an alternativeembodiment, the subscriber station stores a statistic indicative of thepower control commands received from each base station, such as thefraction of commands received that requested a decrease in transmissionenergy in a predetermined number of preceding frames or the number ofdiscrepancies between the base station's request and the response takenby the subscriber station.

In block 104, the subscriber station makes an initial selection of thebase station to transmit forward link data to it. In the exemplaryembodiment, the subscriber station measures the energy of a timemultiplexed pilot signal from each base station and selects the basestation with the highest chip energy to interference (C/I) whenincluding all multipath components. In the exemplary embodiment, thesubscriber station includes a RAKE receiver that separately demodulatesthe multipath components of signals from each base station. An exemplaryembodiment of a RAKE receiver is described in U.S. Pat. No. 5,103,390.

In block 106, the subscriber station determines whether the selectedbase station requires a handoff. That is to say, whether the selectedbase station is the same as the base station selected to transmit in thelast frame interval.

If the selected base station does require a handoff, then the processmoves to block 108. In block, 108, the subscriber determines if theselected base station is receiving its reverse link transmissions. Inthe exemplary embodiment, the subscriber station makes thisdetermination by looking at the history of reverse link power controlcommands transmitted by the selected base station. A sufficient numberof power control commands requesting the subscriber station to decreaseits transmission energy is indicative that the signal strength of itsreverse link transmissions are being received by the selected basestation. It will be understood that other methods of performing thisanalysis are equally applicable, for example the base stations couldintermittently transmit a message indicating the average quality of thereceived reverse link signal. Furthermore, the busy tones, as describedbelow, can be used to determine the quality of the reverse link.

If the subscriber station determines that its reverse link signal isbeing reliably received by the selected base station, then the processmoves to block 110. In block 110, the handoff is permitted. Thesubscriber station transmits a data rate control (DRC) messageindicative of the selected base station and the rate requested totransmit to the subscriber station.

If the subscriber station determines that its reverse link signal is notbeing reliably received by the selected base station, then the processmoves to block 112. In block 112, the handoff is inhibited. In theexemplary embodiment, the subscriber station selects an alternative basestation for transmission of forward link traffic data which is reliablyreceiving its reverse link transmissions. The subscriber stationtransmits a DRC message indicative of the alternative base station andthe rate requested to transmit to the subscriber station. The raterequested is based on the strength of the received pilot signal from thealternative base station.

Back in block 106, if a handoff is not necessary, then the process movesto block 114. In block 114, the subscriber station again determineswhether the selected base station (which is the base station previouslyselected) is receiving its reverse link signal with sufficient energy.The determination as to whether the base station is reliably receivingthe reverse link signal proceeds as described with respect to block 108above.

If the subscriber station determines that its reverse link signal isbeing received with sufficient energy by the selected base station, thenthe process moves to block 116. In block 116, the subscriber stationtransmits a message indicative of the selected base station and the raterequested to transmit to the subscriber station.

If the subscriber station determines that its reverse link signal is notbeing reliably received by the selected base station, then the processmoves to block 118. In block 118, a handoff is forced. The subscriberstation selects an alternative base station for transmission of forwardlink traffic data, which is receiving its reverse link transmissionswith sufficient energy. The subscriber station transmits a DRC messageindicative of the alternative base station and the rate requested totransmit to the subscriber station. The rate requested is based on thestrength of the received pilot signal from the alternative base station.

Network Description

Referring to the figures, FIG. 2 represents an exemplary embodiment of adata communication system comprising multiple cells 200A-200F. Each cell200 is serviced by a corresponding base station 202 or base station 204.Base stations 202 are base stations that are in active communicationwith subscriber station 206 and are said to make up the active set ofsubscriber station 206. Base stations 204 are not in communication withsubscriber station 206 but have signals with sufficient strength to bemonitored by subscriber station 206 for addition to the active set ifthe strength of the received signals increases due to a change in thepropagation path characteristics. Base stations 204 are said to make upthe candidate set of subscriber station 206.

In the exemplary embodiment, subscriber station 206 receives informationfrom at most one base station 202 on the forward link at each time slot,but can be in communication with one or more base stations 202 on thereverse link, depending on whether the subscriber station 206 is in softhandoff. As shown by FIG. 1, each base station 202 preferably transmitsdata to one subscriber station 206 at any given moment. Subscriberstations 206, especially those located near a cell boundary, can receivethe pilot signals from multiple base stations 204 in the candidate set.If the pilot signal is above a predetermined threshold, subscriberstation 206 can request that base station 204 be added to the active setof subscriber station 206. In the exemplary embodiment, subscriberstation 206 can receive data transmission from zero or one member of theactive set.

Forward Link Structure

A block diagram of an exemplary embodiment of a base station is shown inFIGS. 3A and 3B. The data is partitioned into data packets and providedto CRC encoder 312. For each data packet, CRC encoder 312 generatesframe check bits (e.g., the CRC parity bits) and inserts the code tailbits. The formatted packet from CRC encoder 312 comprises the data, theframe check and code tail bits, and other overhead bits which aredescribed below. The formatted packet is provided to encoder 314 which,in the exemplary embodiment, encodes the data in accordance with aconvolutional or turbo encoding format. The encoded packet from encoder314 is provided to interleaver 316 which reorders the code symbols inthe packet. The interleaved packet is provided to frame puncture element318 which removes a fraction of the packet in the manner describedbelow. The punctured packet is provided to multiplier 320 whichscrambles the data with the scrambling sequence from scrambler 322. Theoutput from multiplier 320 comprises the scrambled packet. The scrambledpacket is provided to variable rate controller 330 which demultiplexesthe packet into K parallel in-phase and quadrature-phase channels, whereK is dependent on the data rate. In the exemplary embodiment, thescrambled packet is first demultiplexed into the in-phase (I) andquadrature-phase (Q) streams. In the exemplary embodiment, the I streamcomprises even indexed symbols and the Q stream comprises odd indexedsymbol.

Each stream is further demultiplexed into K parallel channels such thatthe symbol rate of each channel is fixed for all data rates. The Kchannels of each stream are provided to Walsh cover element 332 whichcovers each channel with a Walsh function to provide orthogonalchannels. The orthogonal channel data is provided to gain element 334which scales the data to maintain a constant total-energy-per-chip (andhence constant output power) for all data rates. The scaled data fromgain element 334 is provided to multiplexer (MUX) 360 which multiplexesthe data with a preamble sequence. The output from MUX 360 is providedto multiplexer (MUX) 362 which multiplexes the traffic data, the powercontrol bits, and the pilot data. The output of MUX 362 comprises the IWalsh channels and the Q Walsh channels.

The reverse link power control (RPC) bits are provided to symbolrepeater 350 which repeats each RPC bit a predetermined number of times.The repeated RPC bits are provided to Walsh cover element 352 whichcovers the bits with the Walsh covers corresponding to the RPC indices.The covered bits are provided to gain element 354 which scales the bitsprior to modulation so as to maintain a constant total transmit power.

In addition, a forward activity bit is provided to symbol repeater 350.The forward activity bit alerts subscriber station 206 to a forthcomingblank frame in which the base station will not transmit forward linkdata. This transmission is made in order to allow subscriber station 206to make a better estimate of the C/I of the signal from base stations202. The repeated versions of the forward activity bit are Walsh coveredin Walsh cover element 352 so as to be orthogonal to the Walsh coveredpower control bits. The covered bits are provided to gain element 354which scales the bits prior to modulation so as to maintain a constanttotal transmit power.

In addition, a busy tone is provided to symbol repeater 350. The busytone alerts subscriber station 206 to a reverse link loading condition.In an exemplary embodiment, the busy tone is a single bit indicative ofthe reverse link being fully loaded or having capacity. In the preferredembodiment, the busy tone is a two bit signal indicative of a request bybase stations 202 for subscriber stations 206 in its coverage area toeither deterministically increase or decrease the rate of their reverselink transmissions, or to stochastically increase or decrease the rateof their reverse link transmissions. The repeated versions of the busytone is Walsh covered in Walsh cover element 352 so as to be orthogonalto the Walsh covered power control bits and forward activity bit. Thecovered bit is provided to gain element 354 which scales the bits priorto modulation so as to maintain a constant total transmit power.

The pilot data comprises a sequence of all zeros (or all ones) which isprovided to multiplier 356. Multiplier 356 covers the pilot data withWalsh code W₀. Since Walsh code W₀ is a sequence of all zeros, theoutput of multiplier 356 is the pilot data. The pilot data is timemultiplexed by MUX 362 and provided to the I Walsh channel which isspread by the short PN_(I) code within complex multiplier 366 (see FIG.3B). In the exemplary embodiment, the pilot data is not spread with thelong PN code, which is gated off during the pilot burst by MUX 376, toallow reception by all subscriber stations 206. The pilot signal is thusan unmodulated BPSK signal.

A block diagram of the exemplary modulator used to modulate the data isillustrated in FIG. 3B. The I Walsh channels and Q Walsh channels areprovided to summers 364A and 364B, respectively, which sum the K Walshchannels to provide the signals I_(sum) and Q_(sum), respectively. TheI_(sum) and Q_(sum) signals are provided to complex multiplier 366.Complex multiplier 366 also receives the PN_I and PN_Q signals frommultipliers 378A and 378B, respectively, and multiplies the two complexinputs in accordance with the following equation:

$\begin{matrix}\begin{matrix}{\left( {I_{muli} + {j\; Q_{mult}}} \right) = {\left( {I_{sum} + {j\; Q_{sum}}} \right) \cdot \left( {{PN\_ I} + {j\; {PN\_ Q}}} \right)}} \\{= {\left( {{I_{sum} \cdot {PN\_ I}} - {Q_{sum} \cdot {PN\_ Q}}} \right) +}} \\{{{j\; \left( {{I_{sum} \cdot {PN\_ Q}} + {Q_{sum} \cdot {PN\_ I}}} \right)},}}\end{matrix} & (2)\end{matrix}$

where I_(mult) and Q_(mult) are the outputs from complex multiplier 366and j is the complex representation. The I_(mult) and Q_(mult) signalsare provided to filters 368A and 368B, respectively, which filter thesignals. The filtered signals from filters 368A and 368B are provided tomultipliers 370A and 370B, respectively, which multiply the signals withthe in-phase sinusoid COS(w_(c)t) and the quadrature-phase sinusoidSIN(w_(c)t), respectively. The I modulated and Q modulated signals areprovided to summer 372 which sums the signals to provide the forwardmodulated waveform S(t).

In the exemplary embodiment, the data packet is spread with the long PNcode and the short PN codes. The long PN code scrambles the packet suchthat only the subscriber station 206 for which the packet is destined isable to descramble the packet. In the exemplary embodiment, the pilotand power control bits and the control channel packet are spread withthe short PN codes but not the long PN code to allow all subscriberstations 206 to receive these bits. The long PN sequence is generated bylong code generator 374 and provided to multiplexer (MUX) 376. The longPN mask determines the offset of the long PN sequence and is uniquelyassigned to the destination subscriber station 206. The output from MUX376 is the long PN sequence during the data portion of the transmissionand zero otherwise (e.g. during the pilot and power control portion).The gated long PN sequence from MUX 376 and the short PN_(I) and PN_(Q)sequences from short code generator 380 are provided to multipliers 378Aand 378B, respectively, which multiply the two sets of sequences to formthe PN_I and PN_Q signals, respectively. The PN_I and PN_Q signals areprovided to complex multiplier 366.

The block diagram of the exemplary embodiment of the base station shownin FIGS. 3A and 3B is one of numerous architectures which support dataencoding and modulation on the forward link. Other architectures, suchas the architecture for the forward link traffic channel in the CDMAsystem which conforms to the IS-95 standard, can also be utilized andare within the scope of the present invention.

Forward Link Frame Structure

A diagram of an exemplary embodiment of a forward link frame structureis illustrated in FIG. 4A. The traffic channel transmission ispartitioned into frames which, in the exemplary embodiment, are definedas the length of the short PN sequences or 26.67 msec. Each frame cancarry control channel information addressed to all subscriber stations206 (control channel frame), traffic data addressed to a particularsubscriber station 206 (traffic frame), or can be empty (idle frame).The content of each frame is determined by the scheduling performed bythe transmitting base station 202. In the exemplary embodiment, eachframe comprises 16 time slots, with each time slot having a duration of1.667 msec. A time slot of 1.667 msec is adequate to enable subscriberstation 206 to perform the C/1 measurement of the forward link signal. Atime slot of 1.667 msec also represents a sufficient amount of time forefficient packet data transmission.

In the exemplary embodiment, each forward link data packet comprises1024 or 2048 bits. Thus, the number of time slots required to transmiteach data packet is dependent on the data rate and ranges from 16 timeslots for the 38.4 Kbps rate to 1 time slot for the 1.2288 Mbps rate andhigher.

An exemplary diagram of the forward link slot structure is shown in FIG.4B. In the exemplary embodiment, each slot comprises three of the fourtime multiplexed channels, the traffic channel, the control channel, thepilot channel, and the overhead control channel. In the exemplaryembodiment, the pilot signal is transmitted in two bursts and theoverhead control channel is transmitted on either side of the secondpilot burst. The traffic data is carried in three portions of the slot(402A, 402B and 402C).

The first pilot burst 406A is time multiplexed into the first half ofthe slot by multiplexer 362. The second pilot burst 406B is timemultiplexed into the second half of the slot. One either side of secondpilot burst 406B overhead channel data 408 including the forwardactivity bit, the busy tones and the power control bits are multiplexedinto the slot.

Subscriber Station

FIG. 5 illustrates an exemplary embodiment of the subscriber station206. Forward link signals are received at antenna 500 and providedthrough duplexer 502 to receiver 504. In the exemplary embodiment,receiver 504 is a quaternary phase shift keying (QPSK) receiver. It willbe understood by one skilled in the art that the present invention isequally applicable to any other modulation format such as BPSK or QAM.

The in-phase and quadrature-phase components of the received signal areprovided to PN despreaders 506. In the exemplary embodiment, multiple PNdespreaders 506A-506N are provided. Each of despreaders 506 is capableof demodulating a signal from a different base station in the Active setof subscriber station 206 or a different multipath component of thesignal from a base station.

The PN despread signal is provided to power control command (PCC)demodulator 508. In the exemplary embodiment, PCC demodulator 508performs an FHT on the received power control symbols and determineswhether the base station is requesting subscriber station 206 toincrease or decrease its transmission energy.

The demodulated power control symbols are provided to power controlcommand combiner 516. In the exemplary embodiment, power control commandcombiner 516 soft combines multipath components of the power controlcommand symbols from a single base station and generates a hard estimateof the power control command from each base station. The hard estimatefrom each of the base stations is stored in memory 518. In analternative embodiment, a statistic representing the recent history ofpower control commands from each base station is stored in memory 518.

Then, power control command combiner 516 performs an OR-of-the-downsoperation in which the transmission energy of subscriber station 206 isonly increased if all the power control commands indicate a need toincrease the transmission energy. Power control command combiner 516provides a control signal to transmitter (TMTR) 528 increasing ordecreasing its amplification of the reverse link signal from subscriberstation 206.

The PN despread signals from PN despreaders 506 are also provided topilot demodulators 510. Pilot demodulators 510 despread the pilotsignal. In the exemplary embodiment, the Walsh 0 function is used tospread the pilot signal, and as such pilot demodulators 510 areimplemented as accumulators. The despread pilot signals are provided toenergy calculators 512. Energy calculators 512 compute the energy of thedemodulated pilot bursts. In the exemplary embodiment, this operation isperformed by summing the squares of the demodulated symbol amplitudes.The calculated energy values are provided to control processor 520.

Control processor 520 sums the energies from multipath components of acommon base station and generates and chip energy to interference ratiofor each base station. Control processor 520 then selects the basestation with the highest (C/I) and selects a requested rate for thatbase station. After the base station is selected, the operationdescribed in blocks 106-118 of FIG. 1 is performed by control processor520.

After performing the selection process described with respect to FIG. 1,a signal indicative of the selected base station and a symbol indicativeof the requested rate are provided to spreading element 524. In theexemplary embodiment, the rate request is spread by the signalindicative of the selected base station. This signal is multiplexed withother overhead data such as a reverse rate indicator (RRI) and the pilotsymbols. In the exemplary embodiment, this data is provided on thein-phase component of a transmitted QPSK signal. The reverse linktraffic data is modulated and provided for transmission on thequadrature phase component of the transmitted QPSK signal.

Transmitter 528 upconverts, amplifies, and filters the signal fortransmission. In the exemplary embodiment, transmitter 528 also spreadsthe reverse link signal in accordance with a pseudonoise sequence. Thesignal is provided through duplexer 502 for transmission through antenna500.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A subscriber station, comprising: means for selecting a base stationfor transmission of forward link data; means for determining whether areverse link signal is being reliably received by the base station; andmeans for selecting an alternative base station for the transmission ofthe forward link data if the reverse link signal is not being reliablyreceived by the base station.
 2. The subscriber station of claim 1,wherein the means for determining whether the reverse link signal isbeing reliably received by the base station comprises means foranalyzing a history of reverse link power control commands transmittedby the base station.
 3. The subscriber station of claim 1, wherein themeans for determining whether the reverse link signal is being reliablyreceived by the base station comprises means for intermittentlyreceiving a message from the base station indicating an average qualityof the reverse link signal received at the base station.
 4. Thesubscriber station of claim 1, further comprising: means for determiningthat a handoff to the base station is necessary; and means forinhibiting the handoff to the base station if the reverse link signal isnot being reliably received by the base station.
 5. The subscriberstation of claim 1, further comprising: means for determining that ahandoff to the base station is not necessary; and means for forcinghandoff to the alternative base station if the reverse link signal isnot being reliably received by the base station.
 6. A subscriberstation, comprising: circuitry configured to select a base station fortransmission of forward link data, to determine whether a reverse linksignal is being reliably received by the base station, and to select analternative base station for the transmission of the forward link dataif the reverse link signal is not being reliably received by the basestation.
 7. The subscriber station of claim 6, wherein the circuitryconfigured to determine whether the reverse link signal is beingreliably received by the base station comprises circuitry configured toanalyze a history of reverse link power control commands transmitted bythe base station.
 8. The subscriber station of claim 6, wherein thecircuitry configured to determine whether the reverse link signal isbeing reliably received by the base station comprises circuitryconfigured to intermittently receive a message from the base stationindicating an average quality of the reverse link signal received at thebase station.
 9. The subscriber station of claim 6, further comprisingcircuitry configured to determine that a handoff to the base station isnecessary, and to inhibit the handoff to the base station if the reverselink signal is not being reliably received by the base station.
 10. Thesubscriber station of claim 6, further comprising circuitry configuredto determine that a handoff to the base station is not necessary, and toforce handoff to the alternative base station if the reverse link signalis not being reliably received by the base station.
 11. A method forcommunications by a subscriber station, comprising: selecting a basestation for transmission of forward link data; determining whether areverse link signal is being reliably received by the base station; andselecting an alternative base station for the transmission of theforward link data if the reverse link signal is not being reliablyreceived by the base station.
 12. The method of claim 11, whereindetermining whether the reverse link signal is being reliably receivedby the base station comprises analyzing a history of reverse link powercontrol commands transmitted by the base station.
 13. The method ofclaim 11, wherein determining whether the reverse link signal is beingreliably received by the base station comprises intermittently receivinga message from the base station indicating an average quality of thereverse link signal received at the base station.
 14. The method ofclaim 11, further comprising: determining that a handoff to the basestation is necessary; and inhibiting the handoff to the base station ifthe reverse link signal is not being reliably received by the basestation.
 15. The method of claim 11, further comprising: determiningthat a handoff to the base station is not necessary; and forcing handoffto the alternative base station if the reverse link signal is not beingreliably received by the base station.
 16. A computer-program product,including a computer-readable medium encoded with computer-readableinstructions for: selecting a base station for transmission of forwardlink data; determining whether a reverse link signal is being reliablyreceived by the base station; and selecting an alternative base stationfor the transmission of the forward link data if the reverse link signalis not being reliably received by the base station.
 17. Thecomputer-program product of claim 16, wherein the instructions fordetermining whether the reverse link signal is being reliably receivedby the base station comprise instructions for analyzing a history ofreverse link power control commands transmitted by the base station. 18.The computer-program product of claim 16, wherein the instructions fordetermining whether the reverse link signal is being reliably receivedby the base station comprise instructions for intermittently receiving amessage from the base station indicating an average quality of thereverse link signal received at the base station.
 19. Thecomputer-program product of claim 16, wherein the computer-readablemedium is also encoded with computer-readable instructions fordetermining that a handoff to the base station is necessary, and forinhibiting the handoff to the base station if the reverse link signal isnot being reliably received by the base station.
 20. Thecomputer-program product of claim 16, wherein the computer-readablemedium is also encoded with computer-readable instructions fordetermining that a handoff to the base station is not necessary, and forforcing handoff to the alternative base station if the reverse linksignal is not being reliably received by the base station.